FIG. 1 depicts a simplified schematic of a fiber laser 100. Fiber laser 100 includes a light pump 102, an optical fiber 106 and two reflectors, which, in fiber laser 100, are embodied as respective high-reflectivity and low-reflectivity mirrors 110 and 114. Mirrors 110 and 114 are disposed adjacent to respective fiber end faces 108 and 112, defining a laser cavity 104 therebetween. In other embodiments, the reflectors are implemented as gratings that are formed within the fiber 106. Light pump 102, advantageously a laser diode or diode array, launches light 103 (i.e., photons) into the laser cavity 104. The pump photons stimulate the emission of photons in the fiber 106 providing lasing output 116 at a characteristic wavelength, as described further below.
FIG. 2 shows a double-clad fiber 206 suitable for use as fiber 106 of fiber laser 100. Double-clad fiber 206 comprises single-mode fiber core 208, a multi-mode first cladding 210 surrounding fiber core 208, and a second cladding 212 surrounding first cladding 210. Light from light pump 102 is launched into first cladding 210. In some embodiments, light is launched into first cladding 210 at an end of fiber laser 100, known as "end-pumping," (see FIG. 1). In other embodiments (not shown), light from pump 102 is launched into first cladding through a side of fiber laser 100. In the embodiment depicted in FIG. 2, first cladding 210 has a rectangular cross section, wherein the ratio of the lengths of the long side to the short side is in the range from about 1.5/1 to about 10/1. In other embodiments, the first cladding has a "star" or "D-shaped" cross section. Typically, first cladding 210 comprises pure silica.
The geometry and refractive indices of fiber core 208, first cladding 210 and second cladding 212 are such that a substantial amount of light launched into first cladding 210 is coupled into fiber core 208. Such an arrangement is advantageous since light can be launched into multi-mode cladding, such as first cladding 210, without the high tolerances typically required for launching light directly into a single-mode core, such as fiber core 208.
Fiber core 208, typically 4-8 microns in diameter and comprising silica, is doped with one or more ionized rare-earth element ("active lasing element"), such as Nd.sup.3+, Yb.sup.3+, Tm.sup.3+, and Er.sup.3+. The active lasing element absorbs photons that are coupled into fiber core 208. Absorbing such photons increases the energy state of the active lasing element and causes "population inversion." As electrons in the active lasing element decay to lower energy states, photons are emitted that have a wavelength characteristic of the particular lasing element. In some embodiments, co-dopants, used for modifying the refractive index of the fiber core, are used in conjunction with the active lasing elements.
Second cladding 212 substantially prevents light from leaking from first cladding 210 to the outside environment. Such containment is accomplished by ensuring that the index of refraction of second cladding 212 is significantly lower than that of first cladding 210 (typically about 1.38 vs. 1.465). Second cladding 212, depicted in FIG. 2 as having a circular cross section, is suitably formed from a polymer, such as a fluoropolymer, or a low-index glass.
FIG. 3 depicts additional details regarding the launching of light into fiber laser 100. In one embodiment, light pump 102 launches light 302 into a short connector-free section ("a pigtail") of multi-mode fiber 304 having multi-mode core 306. Fiber pigtail 302 is spliced to high reflector grating 110 and aligned to launch light 302 into first cladding 210.
It is desirable, if not critical, to be able to measure the power launched into a fiber laser in order to monitor and correct for drifts in the diode pump and to accurately test the laser output. Presently, no satisfactory method exists for performing such measurements. In one prior-art approach, a detector is located at the back facet of a diode. Such an approach does not, however, measure drifts in launched power. Rather, it measures the power generated at the source (i.e., the diode). Launched light cannot be measured in this manner because reflecting (back-propagating) the launched light for a measurement by the detector may damage the diode. Moreover, a multimode tap would be needed to route such back-propagating light to the detector. Such taps are not readily available. In another method, "integrating spheres" are used to detect light scattered from a fiber pigtail, such a fiber pigtail 302. Such detected light is disadvantageously not a measure of launch power. Furthermore, such integrating spheres are bulky.
As such, the art would benefit from a method and apparatus for accurately measuring the power launched into a fiber laser.